God bless the internet.
Courtesy of Washington Post, a rare recording of Gandhiji addressing a gathering of Asian leaders, recorded on April 2, 1947. (Via Metafilter). What makes this recording unique is its "date stamp" and the fact that the speech is delivered in English.
WaPo writer Shankar Vedantam provides a good historical background for this little audio gem. You can also watch a very interesting video discussion between him and Rajmohan Gandhi about this recording here. (In the video, Rajmohan Gandhi makes a reference to recordings of his grandfather's post-prayer meeting speeches and how they are lying undigitized in Indian government's archives. So much for Information and Broadcasting.)
The audio quality is slightly patchy at times, but not so much that you can't hear the great man's words. And there are some great words in there. Just listen.